The present invention relates to a heater drive circuit for controllably driving a heater to speed up a buildup temperature elevation of a liquid which is to be heated by the heater and, more particularly, to a heater drive circuit suitable for controllably driving a heater installed in an ink jet printer for controlling the temperature of ink, such that the ink temperature builds up in a short period of time.
An ink jet printer generally uses a heater for maintaining ink to be ejected from a head at an appropriate temperature and, for such a heater, various kinds of heater drive circuits have heretofore been proposed. An example of the prior art heater drive circuits is shown in FIG. 1 and generally designated by the reference numeral 10. The heater drive circuit 10 is constructed to sense an ink temperature and an atmospheric temperature, and maintains the ink temperature at a predetermined level by controlling the operation of a heater 12 based on the sensed temperatures at both the buildup and stationary stages. The heater drive circuit 10 includes a constant current circuit 14, and a series connection 20 of a temperature-sensitive element 16 responsive to heater temperature and a temperature-sensitive element 18 responsive to atmospheric temperature which is connected between the constant current circuit 14 and ground. A comparator 22 has an inverting input connecting to a junction of the constant current circuit 14 and the temperature-sensitive element 16, and a non-inverting input supplied with a first reference voltage Vref.sub.1 predetermined for supplying the heater 12 with power necessary for a buildup. An error amplifier 24 has an inverting input connecting to the inverting input of the comparator 22 and a non-inverting input supplied with a second reference voltage Vref.sub.2 predetermined for supplying the heater 12 with power necessary for a normal or stationary operation, which is performed to maintain the heater 12 at a predetermined temperature.
The outputs of the comparator 22 and the error amplifier 24 are connected to the heater 12 via diodes 26 and 28, respectively. Each of the temperature-sensitive elements 16 and 18 comprises a thermistor or a temperature-sensitive resistor and has a positive temperature characteristic. Assuming that the first reference voltage Vref.sub.1 is preselected to control the temperature of a liquid, such as ink in an ink jet printer, to 60.degree. C., then the comparator 22 also functions to determine whether the ink temperature has reached 60.degree. C.
In operation, let it be assumed that the temperature of the heater 12 in the stationary operating condition is 20.degree. C. and the atmospheric temperature is 5.degree. C. To heat the ink quickly to 60.degree. C., the heater 12 is controlled such that the sum of the heater temperature sensed by the element 16 and the atmospheric temperature sensed by the element 18 becomes 60.degree. C. This will be described with reference to FIG. 2, in which the abscissa indicates the atmospheric temperature axis and the ordinate, the heater temperature axis. A solid line Ta in FIG. 2 represents atmospheric temperatures sensed by the element 18, while a dotted line Th.sub.1 and a dash-and-dot line Th.sub.2 individually represent temperatures of the heater 12 sensed by the element 16. Assuming that the atmospheric temperature is 20.degree. C., the heater temperature will be controlled to become 40.degree. C. as indicated by the atmospheric temperature Ta and the heater temperature Th.sub.1. If the atmospheric temperature is 40.degree. C., on the other hand, the heater temperature will be controlled to 20.degree. C. as indicated by the atmospheric temperature Ta and the heater temperature Th.sub.1.
Generally, in an ink jet printer, a heater for heating ink is positioned remote from a head and, therefore, ink temperature is preselected to be high enough to accommodate a temperature drop which the ink undergoes before reaching the head. This brings about a problematic situation that, because the proportion of the temperature drop of the ink decreases with the elevation of the atmospheric temperature, driving the heater 12 in the manner described above causes overshoot at a buildup of the heater 12. Meanwhile, if the first reference voltage Vref.sub.1 applied to the comparator 22 is selected to be a value which does not cause overshoot at the atmospheric temperature of 10.degree. C., the proportion of the temperature drop of the ink becomes substantial to in turn cause undershoot. It is therefore difficult to attain a fast buildup of the ink temperature. If the control is performed such that the optimum ink temperature is 40.degree. C., difficulty will also be experienced in speeding up the buildup although no overshoot will occur.